Since the advent, in 1840, of Sir Win. Logan's paper “On the Characters of the Beds of Clay immediately below the Coal Seams of South Wales,” underclays have generally been regarded as representing the soils on which grew the vegetation now forming the coal. I am not aware, however, that any definite and detailed hypothesis has been put forward to account for their origin. The superior interest attaching to the question of the origin of coal has apparently overshadowed the less important underclay, and it has been left to fit itself in with any particular hypothesis of the former which happened to be in favour. At the same time, two general views of its origin have been suggested which do not, however, go into any detail as to the exact manner in which it has been brought about.